Priority is claimed to Patent Application Numbers 2000-68728 and 2001-52112, filed in the Republic of Korea on Nov. 18, 2000 and Aug. 28, 2001, respectively, both herein incorporated by reference.
1. Field of the Invention
The present invention relates to an anode thin film for lithium secondary battery, and more particularly, to an anode thin film for a lithium secondary battery having improved charging/discharging cycle life characteristics by using silicon (Si) and silver (Ag) as materials of forming an anode active material layer formed on a current collector.
2. Description of the Related Art
Recently, according to advanced microelectronics industry and development of miniaturized, highly efficient electronic devices and very small sensor devices, small and thin batteries as power sources for driving such devices are highly demanded.
FIG. 1 is a schematic diagram of a conventional thin film battery. Referring to FIG. 1, the thin film battery basically is constructed such that a cathode 12, an electrolyte 14 and an anode 13, each in the form of a thin film, are sequentially stacked on a current collector 11, to have the overall thickness of approximately 10 xcexcm, characterized by the following advantages. In FIG. 1, reference number 15 denotes a protective layer.
The thin-film stacked structure in which the anode is arranged in the vicinity of the cathode, increases a current density, leading to an excellent efficiency of a battery, and reduces moving distances of ions between the two electrodes, facilitating ionic mobility, by which the content of a reactive material can be greatly reduced. Also, since such thin film batteries can be easily fabricated in an arbitrary shape and size, they are expected to be used as promising main power sources for driving very small electronic device, MEMS (Micro Electro Mechanical System) and very small sensors.
The thin film battery fabricated by the same process as in a semiconductor device can be mounted with an electronic circuit on a semiconductor chip, thereby implementing a CMOS (Complementary Metal Oxide Semiconductor) memory chip using the thin film battery as a back-up power source. Also, an unused space of an electronic device can be minimized, thereby maximizing the space utilization efficiency. Batteries having various voltages and capacities can be realized by serial/parallel connection through appropriate design and etching, leading to a variety of applications.
Research into thin film batteries has hitherto focused on fabrication and evaluation of cathode thin films formed of V2O5, LiCoO2 or LiMn2O4, and satisfactory research achievements have been reported. Anode thin films for such batteries are mostly lithium thin films formed by deposition of lithium metal.
Lithium metal having a relatively low melting point of approximately 180xc2x0 C., is liable to be molten due to heat generated during soldering that is performed in a packaging process, which may cause damages to the device. Also, since lithium metal is highly reactive in the air, many handling difficulties are involved and an additional device for isolating the device from moisture and oxygen is necessary, which makes it difficult for lithium metal to be put into practical use as an electrode material of power source for driving very small electronic devices.
Although the use of anode thin films of silicon-tin oxynitride (SITON), silicon dioxide (SnO2) or nitride as well as the lithium thin films have been attempted, irreversible reactions occurring during initial charging/discharging cycles are not properly controlled.
To overcome the problem of a low charge/discharge efficiency of lithium, research into lithium alloy has been carried out. Much attention is being paid to a metal that can form lithium alloys, such as tin (Sn), silicon (Si) or aluminum (Al), as a next generation anode active material. While the anode active material exhibits excellent capacity characteristics with respect to lithium in a low voltage range, a change in volume of the active material due to intercalation/deintercalation of lithium during charging/discharging cycles, causes damages to an anode thin film structure, deteriorating cycle characteristics. In particular, in the thin film battery using a solid electrolyte, adhesion at the interface between an electrode and a current collector is considerably reduced, deteriorating the battery performance. Importantly, it is necessary to develop a material having excellent cycle characteristics without a reduction in capacity due to a irreversible reaction during intercalation or deintercalation of lithium.
To solve the above-described problems, it is a first object of the present invention to provide an anode thin film for a lithium secondary battery having improved charging/discharging characteristics.
To accomplish the object of the present invention, there is provided an anode thin film for a lithium secondary battery having a current collector and an anode active material layer formed thereon, wherein the anode active material layer is a multiple-layer thin film comprising a silicon (Si) layer and a silver (Ag) layer.
Also, to accomplish the object of the present invention, there is provided an anode thin film for a lithium secondary battery having a current collector and an anode active material layer formed thereon, wherein the anode active material layer is a single-layer thin film comprising silicon (Si) and silver (Ag).
The multiple-layer thin film layer comprises Si/Ag/Si layers, Si/Ag/Si/Ag layers, Si/Ag/Si/Ag/Si/Ag layers, Si/Ag/Si/Ag/Si/Ag/Si/Ag layers or Ag/Si/Ag/Si/Ag/Si/Ag layers.
Also, In the anode thin film of the present invention, the topmost layer of the multiple-layer thin film layer is preferably formed of Ag. The lithium battery employing the anode thin film has a better cycle characteristics.
In the anode thin film of the present invention, the Si layer and the Ag layer are alternately stacked. In particular, the Ag layer is formed between Si layers.